We setup just one route that is handled by the cadena_h_keys module, it's a
plain HTTP handler, no fancy REST stuff for now, there we handle the request on
the init/2 function itself, we pattern match against the method field on the
request object and handle:

To avoid having to configure this in sys.config we will define a cuttlefish
schema in config.schema that cuttlefish will use to generate a default config
file and validation code for us.

We have to replace the variables from variable overrides in our config.schema
file for each release before it's processed by cuttlefish itself, for that we
use the template directive on an overlay section on the release config.